Liquid crystal display and method for producing the same

ABSTRACT

There is provided a liquid crystal display which is capable of preventing light from leaking out from the peripheral portion of pixels and which has a high display performance. The liquid crystal display comprises: an array substrate including a plurality of scanning lines and a plurality of signal lines, a plurality of switching elements, a plurality of pixel electrodes, and a first alignment layer which is formed on the first substrate so as to cover the pixel electrodes; a counter substrate including a counter electrode and a second alignment layer which is formed on the second substrate so as to cover the counter electrode; and a light control layer sandwiched between the array substrate and the counter substrate, and including a liquid crystal material having a spontaneous polarization and having a nematic phase or an isotropic phase on a high-temperature side of a chiral smectic C phase, an optical axis of liquid crystal molecules in the light control layer substantially staying when no electric field or a first electric field of a first polarity are applied to said liquid crystal material, and the optical axis of the liquid crystal molecules responding in accordance with a magnitude of a second electric field of a second polarity different from the first polarity when the second electric field is applied to said liquid crystal material, wherein an electric field between the scanning lines and the counter electrode has the first polarity when the switching elements turn on.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of priority under 35USC §119 toJapanese Patent Application No. 2000-91592, filed on Mar. 29, 2000, thecontents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of The Invention

[0003] The present invention relates generally to a liquid crystaldisplay using a liquid crystal having a spontaneous polarization, and amethod for producing the same.

[0004] 2. Description of Related Art

[0005] Liquid crystal displays have characteristics, such as lowelectric power consumption, light weight and thin type, and are widelyused as monitors for personal computers and car navigation systems.However, as compared with CRTs, there are disadvantages in that thespeed of response is slow, the viewing angle is narrow, and so forth.With the scale up and the higher resolution of liquid crystal displays,the requirements for a fast response and a wide angle of visibility areenhanced.

[0006] Liquid crystal displays using spontaneous polarization are widelynoticed as a display mode capable of realizing a fast response. Thespontaneous polarization is inherent in a liquid crystal, or induced byapplying an electric field to the liquid crystal. Examples of suchliquid crystal materials (display modes) include surface stabilizedferroelectric liquid crystal (SS-FLC), monostable ferroelectric liquidcrystal, deformed helix ferroelectric liquid crystal (DHF), twistedferroelectric liquid crystal (twisted FLC), alternating polarizationdomain (APD), polymer stabilized ferroelectric liquid crystal,anti-ferroelectric liquid crystal (including thresholdlessanti-ferroelectric liquid crystal), and electro-clinic effect.

[0007] In order to realize full-color displays by combining the abovedescribed display modes with active elements, it is desired that theswitching of liquid crystal molecules does not produce domains. InInternational Conference on Ferroelectric Liquid Crystal (FLC 99) whichwas open in Germany on August, 1999, the continuous director rotation(CDR) mode was reported. In this mode, it is possible to carry out afull-color half-tone display since the optical axes (long axes) ofliquid crystal molecules continuously rotate in accordance with appliedvoltages.

[0008] In the CDR mode, a smectic phase is formed by applying amonopolar electric field (dc electric field) between pixel electrodesand a counter electrode during the phase transition of the liquidcrystal material from the nematic phase or the isotropic phase to thechiral smectic C phase. At this time, although a desired monopolarelectric field can be applied to a region in which the pixel electrodesface the counter electrode, a desired electric field can not be appliedto a region outside of the pixel electrodes. Therefore, a uniform liquidcrystal can not be obtained outside of the pixel region. After theinventors diligently studied, it was found that the following problem iscaused because of ununiform alignment outside of the pixel region.

[0009] That is, if the liquid crystal is driven at room temperatures for1000 hours or more or at a temperature of 10° C. or lower for 50 hoursor more, the turbulence of alignment around the pixel region ispropagated into pixels, so that light leakage occurs to lower contrast.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to eliminatethe aforementioned problems and to provide a liquid crystal displaywhich can prevent light from leaking out from the peripheral portion ofpixels and which has a high display performance, and a method forproducing the same.

[0011] In order to accomplish the aforementioned and other objects,according to one aspect of the present invention, a liquid crystaldisplay comprises: an array substrate including a plurality of scanninglines and a plurality of signal lines, the scanning lines and the signallines being formed on a first substrate in the form of a matrix, aplurality of switching elements which are formed at points ofintersection between the scanning lines and the signal lines, one end ofeach of the switching elements being connected to a corresponding one ofthe signal lines, and each of the switching elements carrying out aswitching action in response to a signal of a corresponding one of thescanning lines, a plurality of pixel electrodes, each of which isconnected to the other end of a corresponding one of the switchingelements, and a first alignment layer which is formed on the firstsubstrate so as to cover the pixel electrodes; a counter substrateincluding a counter electrode which is formed on a second substrate, anda second alignment layer which is formed on the second substrate so asto cover the counter electrode; and a light control layer sandwichedbetween said array substrate and said counter substrate, and including aliquid crystal material having a spontaneous polarization and having anematic phase or an isotropic phase on a high-temperature side of achiral smectic C phase, an optical axis of liquid crystal molecules insaid light control layer substantially staying when no electric field ora first electric field of a first polarity are applied to said liquidcrystal material, and said optical axis of said liquid crystal moleculesresponding in accordance with a magnitude of a second electric field ofa second polarity different from said first polarity when said secondelectric field is applied to said liquid crystal material, wherein anelectric field between said scanning lines and said counter electrodehas said first polarity when said switching elements turn on.

[0012] The switching elements may be disposed under said pixels.

[0013] A direction of a smectic layer in the light control layerpreferably has a distribution of 10° or less.

[0014] If each of the switching elements has a negative TFT and if asmectic layer is formed by cooling the cell without the application ofvoltage, the first alignment layer has an alignment characteristic thatthe spontaneous polarization of liquid crystal molecules is directed tothe first substrate when no voltage is applied to said liquid crystalmaterial.

[0015] If each of the switching elements has a positive TFT and if asmectic layer is formed by cooling the cell without the application ofvoltage, the first alignment layer has an alignment characteristic thatthe spontaneous polarization of liquid crystal molecules is directed tothe second substrate when no voltage is applied to said liquid crystalmaterial.

[0016] According to another aspect of the present invention, there isprovided a method for producing a liquid crystal display comprising anarray substrate including a plurality of scanning lines and a pluralityof signal lines, the scanning lines and the signal lines being formed ona first substrate in the form of a matrix, a plurality of switchingelements which are formed at points of intersection between the scanninglines and the signal lines, one end of each of the switching elementsbeing connected to a corresponding one of the signal lines, and each ofthe switching elements carrying out a switching action in response to asignal of a corresponding one of the scanning lines, a plurality ofpixel electrodes, each of which is connected to the other end of acorresponding one of the switching elements, and a first alignment layerwhich is formed on the first substrate so as to cover the pixelelectrodes; a counter substrate including a counter electrode which isformed on a second substrate, and a second alignment layer which isformed on the second substrate so as to cover the counter electrode; anda light control layer which is sandwiched between the array substrateand the counter substrate and which is made of a liquid crystal materialhaving a spontaneous polarization and having a nematic phase or anisotropic phase on a high-temperature side of a chiral smectic C phase,the method comprising: forming a chiral smectic C phase with applying anelectric field of a polarity between the pixel electrodes and thecounter electrode when a phase transition of the liquid crystal materialfrom a nematic phase or an isotropic phase to the chiral smectic C phaseoccurs, wherein said polarity of said electric field equals to apolarity of an electric field between the counter electrode and thescanning lines when the switching elements turn on.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be understood more fully from thedetailed description given herebelow and from the accompanying drawingsof the preferred embodiments of the invention. However, the drawings arenot intended to imply limitation of the invention to a specificembodiment, but are for explanation and understanding only.

[0018] In the drawings:

[0019]FIG. 1 is a schematic diagram showing the construction of thefirst preferred embodiment of a liquid crystal display according to thepresent invention;

[0020]FIG. 2 is a graph showing the relationship between appliedvoltages and light transmittance in the first preferred embodiment;

[0021]FIG. 3 is a schematic diagram for explaining advantages in thefirst preferred embodiment;

[0022]FIG. 4 is a schematic diagram showing the construction of thesecond preferred embodiment of the present invention;

[0023]FIG. 5 is a schematic diagram showing the construction of thethird preferred embodiment of the present invention;

[0024]FIG. 6 is a schematic diagram for explaining advantages in thefourth preferred embodiment of the present invention;

[0025]FIG. 7 is a schematic diagram showing a comparative example of thefourth preferred embodiment;

[0026]FIG. 8 is a schematic diagram showing the construction of thesixth preferred embodiment of the present invention;

[0027]FIG. 9 is a schematic diagram for explaining the structure whereinpixels are arranged upwards; and

[0028]FIG. 10 is a schematic diagram showing the construction of thefourth preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Referring now to the accompanying drawings, the preferredembodiments of the present invention will be described below.

[0030] (First Preferred Embodiment)

[0031] Referring to FIGS. 1 through 3, the first preferred embodiment ofa liquid crystal display according to the present invention will bedescribed below. The liquid crystal display in this preferred embodimentis an active matrix driving liquid crystal display.

[0032]FIG. 1(a) is a plan view of an active matrix driving liquidcrystal display element in this preferred embodiment, and FIG. 1(b) is asectional view taken along line A-A′ of FIG. 1(a).

[0033] As shown in FIGS. 1(a) and 1(b), the liquid crystal displayelement in this preferred embodiment comprises an array substrate 10, acounter substrate 30, and a light control layer 40 of a liquid crystalmaterial which is sandwiched between the substrates so as to have apredetermined thickness by spacers 45. This liquid crystal material hasa nematic phase or an isotropic phase on the high temperature side of achiral smectic C phase and has a spontaneous polarization.

[0034] The array substrate 10 has a transparent insulating substrate 11.On the principal surface of the substrate 11, a plurality of scanninglines (gate lines) 12 and auxiliary capacitive lines (not shown), whichextend in one direction, are formed. A transparent insulating layer 14is formed on the principal surface of the substrate 11 so as to coverthe scanning lines 12 and the capacitor lines (see FIG. 1(b)). On theinsulating layer 14, a plurality of pixel electrodes 15 of ITO (IndiumTin Oxide) are formed, and a plurality of signal lines 16 are formed soas to be substantially perpendicular to the scanning lines 12 (see FIGS.1(a) and 1(b)). The signal lines 16 are covered with an insulating film17 (see FIG. 1(b)). On the principal surface of the substrate 11 in thevicinity of the points of intersection between the scanning lines 12 andthe signal lines 16, switching elements 18 of TFTs are formed. The gateof each of the switching elements 18 is connected to a corresponding oneof the scanning lines 12. One terminal of the source and drain of eachof the switching elements 18 is connected to a corresponding one of thesignal lines 18 via a contact (not shown) provided in the insulatingfilm 17, and the other terminal is connected to a corresponding one ofthe pixel electrodes 15 via a contact (not shown) provided in theinsulating film 17.

[0035] An alignment layer 19 is formed on the principal surface of thesubstrate 11 so as to cover the pixel electrodes 15 and the switchingelements 18. On the reverse surface of the substrate 11, a polarizer 28is formed.

[0036] On the other hand, the counter substrate 30 is provided with acolor filter portion 32. The color filter portion 32 comprises a colorportion 32, formed in a pixel region on the principal surface of atransparent insulating substrate 31, for transmitting light beams havinga specific wavelength, and black matrixes 32 b which is formed in anon-pixel region on the principal surface of the transparent insulatingsubstrate 31. On the display region of the color filter portion 32, acounter electrode 34 of ITO is formed. An alignment layer 36 is formedon the counter electrode 34 via an inorganic insulating film 35. Theinorganic insulating film 35 is preferably provided in order to maintaininsulating properties. On the reverse surface of the substrate 31, apolarizer 38 is formed.

[0037] The optical axis 28 a of the polarizer 28 of the array substrate10 and the optical axis 38 a of the polarizer 38 of the countersubstrate 30 are arranged so as to form a crossed Nicols configuration(see FIG. 1(a)).

[0038] In this preferred embodiment, alignment treatments, such asrubbing, are carried out on the alignment layers 19 and 38. For example,as shown in FIG. 1(a), an alignment treatment 54 is carried out on thealignment layer 19 of the array substrate 10 in directions of the gatelines 12. In FIG. 1(a), reference number 50 denotes liquid crystalmolecules, and cones shown in FIG. 1(a) denote the locus of the liquidcrystal molecules 50 when a voltage is applied.

[0039] A semiconductor thin film constituting the TFT 18 may be made ofan amorphous silicon or a polysilicon. A polysilicon TFT using apolysilicon is suited to switch a liquid crystal having a spontaneouspolarization since it has a high mobility. The polysilicon TFT caneasily prepare a negative TFT. Furthermore, the negative TFT means aTFT, the gate of which is in ON state (low impedance) when the potentialof the gate electrode is low compared with the potentials of the sourceelectrode and the drain electrode. The amorphous silicon TFT isgenerally a positive TFT for production reasons.

[0040] If a leveling film is formed between the counter electrode 34 andthe color filter portion 32, the counter electrode 34 is flattened, sothat the alignment properties of the liquid crystal is improved and thecounter electrode 34 and the array substrate 10 are difficult to beshort-circuited.

[0041] The leveling film is preferably formed of any one of organicfilms, such as acrylic, polyimide, nylon, polyamide, polycarbonate,benzocyclobutene polymer, polyacrylonitrile and polysilane films.Acrylic films in view of costs, benzocyclobutene polymer films in viewof planarization characteristics, and polyimide films in view ofchemical stability are more preferred.

[0042] The switching element 18 may also be a thin film diode (TFD)element or the like, not the TFT element, if it can switch thecorresponding pixel. The color filter may also be formed on the side ofthe array substrate.

[0043] The array substrate 10 and the counter substrate 30 are adheredto each other by a sealing material, which is applied on the non-displayregion, so that the alignment layers 19 and 36 face each other, exceptfor a filling inlet (not shown). At this time, the distance between thearray substrate 10 and the counter substrate 30 is held to be apredetermined distance by means of the spacers 45.

[0044] The liquid crystal material 40 is introduced by a filling processfor introducing the liquid material 40 via the filling inlet after theinterior of the cell is evacuated. The filling inlet is completelysealed by a saling material (not shown) after filling the liquid crystalmaterial, and isolated from the outside air.

[0045] In the liquid crystal cell thus formed, the liquid crystal 40 isheated to the isotropic phase or the nematic phase. Then, the liquidcrystal 40 is cooled to cause the phase transition of the liquid crystal40 from the nematic phase to the chiral smectic C phase. At this time, amonopolar electric field is applied between the counter electrode 34 andthe pixel electrode 15 on the basis of the potential of the counterelectrode 24. By thus forming the chiral smectic C phase, it is possibleto uniformly orient the liquid crystal molecules 50 sandwiched betweenthe counter electrode 34 and the pixel electrode 15. As shown in FIG.1(b), when no voltage is applied, the molecular orientation of theliquid crystal is substantially parallel to the rubbing direction.

[0046] As shown in FIG. 2, in the voltage-transmittance characteristics,when the above described monopolar electric field is applied, the majoraxes of the liquid crystal molecules 50 hardly vary to be arrangedsubstantially at the same position as that when no voltage is applied,and when the opposite polar electric field to the above describedmonopolar electric field is applied, the major axes of the liquidcrystal molecules 50 vary in accordance with the magnitude of theelectric field.

[0047] After the inventors diligently studied, it was found that thefollowing embodiments were preferred according to the present invention.

[0048] Assuming that the chiral pitch of the liquid crystal 40 is p andthe distance between the array substrate 10 and the counter substrate 30is d, it is preferred that d<p. If d<p, it is possible to prevent theliquid crystal from having a twisted structure. If the liquid crystal 40has the twisted structure, the light passing increases when no voltageis applied, and the contrast of the liquid crystal display is lowered.

[0049] In the working temperature range (usually 0° C.˜50° C.) of theliquid crystal display, when the apparent tilt angle (actually measuredtilt angle) of the liquid crystal molecules 50 is 22.5° or more, theangle between the optical axis of the liquid crystal and thetransmission angle of the polarizer is 45° or more when the spontaneouspolarization is reversed by the application of voltage, consequently thetransmittance becomes maximum. Therefore, in order to obtain a highcontract liquid crystal display, the apparent tilt angle of the liquidcrystal molecules 50 is preferably about 22.5° or more.

[0050] As the materials of the alignment layer for use in the liquidcrystal display in this preferred embodiment, there may be used organicfilms, such as acrylics, polyimides, polyamides, polycarbonates,polyacrylonitriles, polysilanes, polyamic acids, polyetheramides,polyamideimides, nylons, and benzocyclobutene polymers, and obliquelyevaporated silicon oxides. In view of the facility of formation andchemical stability, polyimides and polyacrylonitriles are particularlypreferred.

[0051] If the alignment layer is made of a polyimide, the polyimidepreferably has a relatively low polarity (relatively strong hydrophobicproperty). For example, such polyimides include polyimides havingimidizing rate of 85% or more, polyimides containing fluorine atoms (CF₃group), polyimides having a benzene ring at acid anhydride portionsthereof, polyimides having no oxygen atoms (ether linkages) at diamineportions thereof, and polyimides having —CH₂— bonds at diamine portionsthereof. The reason why polyimides having relatively low polarities aresuitable for the present invention will be described below.

[0052] When the smectic phase appears, the polarity surface interactionbetween the liquid crystal molecules 50 and the alignment layer appliesforce so that the spontaneous polarization of the liquid crystalmolecules 50 is directed to the outside (or inside) (whether outside orinside is determined by the electron affinity of alignment layer). Whenthis force collides with the dc voltage applied between the counterelectrode 34 and the pixel electrode 15 (for example, when thespontaneous polarization is oriented to the inside by the dc voltagealthough the spontaneous polarization is intended to be directed to theoutside by the polarity surface interaction on the interface of thearray substrate 10), the degree of orientation of the liquid crystallowers. In order to prevent this, the polarity surface interactionshould be small. Since the polarity surface interaction between thepolyimide alignment layer and the liquid crystal is small as thepolarity of the polyimide alignment layer is small, low polaritypolyimides are suitable for a liquid crystal display according to thepresent invention.

[0053] With respect to the alignment layers suitable for the presentinvention, their materials and rubbing conditions for applying arelatively low pretilt angle (4° or less) to the liquid crystal arepreferred. The reason for this is that as the pretilt angle decreases,the anchoring force between the liquid crystal molecules and the surfaceof the alignment layer increases, so that the orientation of the liquidcrystal can be more uniform. The rubbing directions of the arraysubstrate 10 and the counter substrate 30 are preferably anti-parallelrather than parallel to each other. The anti-parallel rubbing moreeasily provides a bookshelf structure or a tilted bookshelf structure.As a result, there are no region of alignment defects such as zigzagdefects, so that good alignment characteristics can be obtained.

[0054] When the smectic layer 52 is formed, the dc voltage or offsetvoltage to be applied to the cell is preferably in the range of from 0.2V to 10 V. Because there are some cases where the spontaneouspolarization is not directed in one direction if the voltage is lessthan 0.2 V, and there are some cases where ionic impurities contained inthe liquid crystal 40 are absorbed onto the surface of the alignmentlayer to cause image sticking defects if the voltage is higher than 10 Vwhen the smectic layer 52 is formed if the voltage is higher than 10 V.

[0055] When the smectic layer 52 is formed, the following method forapplying a dc voltage is preferred. Referring to FIG. 3, assuming thatthe reference potential of the signal lines (Vsig, center) is 0 V, thismethod will be described below. Generally, the maximum voltage capableof being applied to the signal lines 16 is only ±7 V due to thewithstand voltage of the driver IC. Although there is no limit to thevoltage applied to the counter substrate 34, the voltage is preferably10 V or less as described above. In FIG. 3, there is used an example ofthe gate voltage applied to the gate lines 12 when the TFT 18 is turnedon, and it is assumed that the gate voltage is −20 V in the case of anegative TFT. In a case where a polyimide film is used as the alignmentlayer, liquid crystal molecules tend to be oriented so that thespontaneous polarization 56 of the liquid crystal molecules 50 isdirected to the outside of the substrate by the polarity surfaceinteraction (electroclinic effect) between the polyimide alignment layerand the liquid crystal material 40 unless the external electric field isapplied when the smectic C phase is formed. As shown in FIGS. 3(a)through 3(d), voltages are applied to the crystal sandwiched between thesignal lines and the counter electrode and between the pixel electrodesand the counter electrode. But, since there is no electrode between thegate lines and the pixel electrodes, no electric field is applied to thecrystal sandwiched between a portion, wherein no electrode is providedbetween the gate lines and the pixel electrodes, and the counterelectrode when the smectic C phase is formed. Therefore, the spontaneouspolarization 56 is directed to the array substrate as described above.In FIGS. 3(a) and 3(b), the direction of the electric field is the sameas this direction, and the light leakage in the peripheral portion ofthe pixels is minimum as compared with other cases shown in FIGS. 3(c)and 3(d). Therefore, in a case where the polyimide alignment film isused, the negative TFT is preferably used, and the voltage is preferablyapplied in accordance with the relationship of voltage shown in FIG.3(a) or 3(b). Moreover, when a high voltage is applied, the voltage ispreferably applied in accordance with the relationship shown in FIG.3(a). Furthermore, if the TFT 18 is a positive TFT in this preferredembodiment, the direction of the spontaneous polarization 56 of theliquid crystal sandwiched between a portion, wherein no electrode isprovided between the gate lines and the pixel electrodes, and thecounter electrode is different from the direction of the electric field,so that the light leakage in the peripheral portion of the pixels isgreater than that in this preferred embodiment.

[0056] If the completed liquid crystal display is observed by amicroscope or the like while it is driven, it is possible to confirm thepresence of light leakage in the vicinity of the gate lines 12.Moreover, if the output of the driver IC is monitored by a probe or thelike, it can be determined if the electric field, which is causedbetween the gate lines 12 and the counter electrode 34 when theswitching element 18 is turned on, has the same polarity as that of theelectric field on the side wherein the axes of the liquid crystalmolecules 50 hardly vary.

[0057] As described above, according to this preferred embodiment, it ispossible to prevent light from leaking out from the peripheral portionof the pixels, so that it is possible to obtain a liquid crystal displayhaving a high display performance.

[0058] A method for producing the first preferred embodiment of a liquidcrystal display according to the present invention will be describedbelow.

[0059] First, TFT element 18 s are formed on a glass substrate 11 asfollows.

[0060] Capacitor lines of chromium (not shown) and gate lines 12 wereformed on the glass substrate 11. The capacitor lines and the gate lines12 were covered with an insulating film 14 having a stacked structurecomprising a chromium oxide film and a silicon oxide film, and asemiconductor layer (not shown) of amorphous silicon was patterned onthe insulating film 14. On the semiconductor layer, a channel protectivelayer (not shown) of silicon nitride was formed. On the semiconductorlayer and the channel protective layer, source electrodes electricallyconnected to the semiconductor layer via a ohmic layer, and drainelectrodes integral with signal lines 16 were formed. Moreover, pixelelectrodes 15 electrically connected to the source electrodes wereformed. Thus, the TFT elements 18, the signal lines 16, the gate lines12 and the pixel electrodes 15 are formed on the glass substrate 11.

[0061] In order to prevent the short-circuit to the counter electrode34, the TFT elements 18, the signal lines 16, the gate lines 12 and thepixel electrodes 15 were covered with a silicon oxide film (not shown)having a thickness of 100 nm.

[0062] A color filter 32 a and a counter electrode 34 were formed on theglass substrate 31 as follows.

[0063] By patterning a chromium film on the glass substrate 31, a blackmatrix 32 b was formed. A color filter film of a photosensitive acrylicresin, in which pigments of red, green and blue were mixed, was formedthereon. Moreover, a transparent acrylic resin was applied thereon as aleveling film (not shown). On the leveling film, a counter electrode 34of ITO was formed by sputtering.

[0064] After the array substrate on which the TFT elements 18 have beenformed, and the counter substrate on which the counter electrode 34 hasbeen formed were cleaned, a polyimide solution (SE-5291 produced byNissan Chemical Industries, Ltd., γp: 6 dyn/cm) was applied on thesesubstrate by the offset printing. Using a hot plate, this was burned at90° C. for 1 minute, and then, at 180° C. for 10 minutes to providealignment layers 19 and 36.

[0065] Then, a rubbing treatment was carried out on the alignment layers19 and 36 on the array substrate 10 and counter substrate 30 by using ofcloth of cotton. The rubbing direction is shown in FIG. 1(a). A rubbingcloth of cotton having piles having a diameter of 0.1 to 10 microns wasused. As rubbing conditions, the revolving speed of a rubbing roller was500 rpm, the moving speed of the substrate was 20 mm/s, the pushingdepth was 0.7 mm, and the number of rubbing operations was one.

[0066] After the rubbing, the alignment layers 19 and 36 on the arraysubstrate 10 and counter substrate 30 were cleaned with an aqueoussolution containing a neutral surfactant as a principal component, toremove contamination adhering to the alignment layers from the rubbingcloth.

[0067] Then, spacer particles 45 (diameter: 2.0 μm) of silicon oxide(SiO₂) were distributed on the alignment layer 19 of the array substrate10. In addition, a sealing material of an epoxy resin was printed on theperipheral portion of the counter substrate 30 by means of a dispenser.

[0068] The surfaces of the array substrate 10 and counter substrate 30,on which the alignment layers were formed, were directed inside to faceeach other. The array substrate 10 and the counter substrate 30 werealigned, and the sealing material was heated to 160° C. in a pressurizedstate to be cured to form a cell. Furthermore, the rubbing directions onthe array substrate 10 and counter substrate 30 were anti-parallel toeach other.

[0069] After this cell was put in a vacuum chamber to be in vacuum, aferroelectric liquid crystal composition 40 (phase series: solidphase→30° C.→chiral smectic C phase→80° C. →nematic phase→85°C.→isotropic phase, tilt angle at 30° C.: 22.5°, spontaneousplanarization: −7 nC/cm²) was injected into the cell via a fillinginlet. However, when the liquid crystal was injected, the cell and theliquid crystal 40 were heated to 100° C. Thereafter, the filing inletwas sealed with an epoxy adhesive.

[0070] Then, the extracted portions of the signal lines 16, gate lines12, capacitor lines and counter electrode 34 of the cell filled with theliquid crystal 40 were connected to terminals, to which voltages are tobe applied, via an anisotropic conductive film. Then, the cell washeated to 90° C. in an oven. A voltage of −20 V was applied to the gatelines 12 to cause the TFT elements 18 to be always in ON state, and avoltage of 0 V was applied to the signal lines 16 to hold the pixelelectrode 15 at 0 V. In addition, a voltage of 0 V was applied to thecapacitor lines, and a voltage of +8 V was applied to the counterelectrode 34. While these voltages are applied, the cell was cooled from90° C. to 25° C. at a rate of 1° C./min to form a smectic layer 52.

[0071] After this cell was observed by a polarizing microscope, thesmectic layer 52 was the same as that shown in FIG. 1(a).

[0072] After the gap of this cell was measured, it was 2.0 μm. Thechiral pitch of the liquid crystal used in this preferred embodiment was4.0 μm which was longer than the cell gap. Therefore, the liquid crystaldid not have a twisted alignment.

[0073] Then, a set of polarizer 28 and 38 were applied on the outside ofthe cell. Furthermore, the transmitting axis of one polarizer 38 wasparallel to the optical axis of the liquid crystal molecules 50 when novoltage was applied, and the transmitting axis of the other polarizer 28was perpendicular to the optical axis of the liquid crystal molecules 50when no voltage was applied. On the cell on which the polarizers wereapplied, a driving circuit, such as a driver IC, was mounted, and a backlight and so forth were mounted to complete a liquid crystal display inthis preferred embodiment.

[0074] In this case, since the direction of the spontaneous polarizationof the liquid crystal of portions between the gate lines 16 and thepixel electrodes 15 and between the signal lines 16 and the pixelelectrodes 16 were the same as that of the liquid crystal sandwichedbetween the pixel electrodes and the counter electrode, light was hardlyleaked out from the non-pixel portion, so that it was possible to obtaina contrast of 300:1. In addition, the viewing angle (a region having acontrast of 10:1 or more and no reversal of gray scales) was 70° or morein vertical and horizontal directions, and the alignment characteristicsand contrast were not deteriorated after driving tests at 0° C., 25° C.and 50° C. for 3000 hours. While the spacers have been distributed inthis preferred embodiment, post-like or wall-like spacing means may beformed on the alignment layer by a photolithography process in place ofthe spacers. In this case, the spacing means is preferably formed on thegate lines so that the alignment defects caused by the rubbing can behidden with the gate lines.

[0075] (Second Preferred Embodiment)

[0076] Referring to FIG. 4, the second preferred embodiment of a liquidcrystal display according to the present invention will be describedbelow. In this second preferred embodiment, a rubbing direction 54 andthe transmitting axes of polarizers 28 and 38 are different from thosein the first preferred embodiment shown in FIG. 1. Other constructionsare the same as those in the first preferred embodiment. In the secondpreferred embodiment, the rubbing direction 54 is substantiallyanti-parallel to signal lines 16. The transmitting axis 38 a of thepolarizer 38 is parallel to the signal lines 16.

[0077] After the liquid crystal display in this preferred embodiment wasactually produced and its performance was measured, it was possible toobtain the same performance as that in the first preferred embodiment.

[0078] (Third Preferred Embodiment)

[0079] The construction of the third preferred embodiment of a liquidcrystal display according to the present invention is shown in FIG. 5.In this third preferred embodiment, a rubbing direction 54 and thetransmitting axes 28 a and 38 a of polarizer 28 and 38 are differentfrom those in the first preferred embodiment shown in FIG. 1. Otherconstructions are the same as those in the first preferred embodiment.

[0080] In the third preferred embodiment, there is a predetermined angleθ (0<θ<90° ) between the rubbing direction 54 and gate lines 12, andthere is a predetermined angle θ between the transmitting axis 38 a ofthe polarizer 38 and the gate lines 12. In this preferred embodiment,the rubbing direction was determined so as to form a layer substantiallyparallel to the gate lines.

[0081] After the liquid crystal display in this preferred embodiment wasactually produced and its performance was measured, it was possible toobtain the same performance as that in the first preferred embodiment.

[0082] (Fourth Preferred Embodiment)

[0083] The construction of the fourth preferred embodiment of a liquidcrystal display according to the present invention is shown in FIG. 10.In the liquid crystal display in this fourth preferred embodiment, TFTsconstituting switching elements 18 are positive TFTs in place of thenegative TFTs of the liquid crystal display in the first preferredembodiment. Therefore, for reasons which will be described later,alignment layers 19 and 36 are made of a nylon. Furthermore, abenzocyclobutene polymer may be used in place of the nylon.

[0084] The liquid crystal display in the fourth preferred embodiment isformed as follows.

[0085] The above described material was used for forming a cell filledwith the liquid crystal, in the same manner as that in the firstpreferred embodiment.

[0086] Then, the extracted portions of the signal lines 16, gate lines12, capacitor lines and counter electrode 34 of the cell filled with theliquid crystal 40 were connected to terminals, to which voltages are tobe applied, via an anisotropic conductive film. Then, the cell washeated to 90° C. in an oven. As shown in FIG. 6(a), a voltage of +20 Vwas applied to the gate lines 12 to cause the TFTs 18 to be always in ONstate, and a voltage of +7 V was applied to the signal lines 16 to holdthe pixel electrode 15 at +7 V. In addition, a voltage of +7 V wasapplied to the capacitor lines, and a voltage of 0 V was applied to thecounter electrode 34. While these voltages are applied, the cell wascooled from 90° C. to 25° C. at a rate of 1° C./min to form a smecticlayer 52.

[0087] After this cell was observed by a polarizing microscope, thesmectic layer 52 was the same as that shown in FIG. 10.

[0088] After the gap of this cell was measured, it was 2.0 μm. Thechiral pitch of the liquid crystal 40 used in this preferred embodimentwas 4.0 μm which was longer than the cell gap. Therefore, the liquidcrystal did not have a twisted alignment.

[0089] Then, a set of polarizer 28 and 38 were applied on the outside ofthe cell. Furthermore, the transmitting axis 38 a of one polarizer 38was parallel to the optical axis of the liquid crystal molecules 50 whenno voltage was applied, and the transmitting axis 28 a of the otherpolarizer 28 was perpendicular to the polarizer 38. On the cell on whichthe polarizers were applied, a driving circuit, such as a driver IC, wasmounted, and a back light and so forth were mounted to complete a liquidcrystal display in this preferred embodiment.

[0090] This liquid crystal display hardly leaks light out from thevicinity of the gate lines, so that it was possible to obtain a frontcontrast of 300:1. In addition, the viewing angle (a region having acontrast of 10:1 or more and no reversal of gray scales) was 70° or morein vertical and horizontal directions, and the alignment characteristicsand contrast were not deteriorated after driving tests at 0° C., 25° C.and 50° C. for 3000 hours.

[0091] In the fourth preferred embodiment, since the alignment layers 19and 36 are made of a nylon or a benzocyclobutene polymer, the polaritysurface interaction (electroclinic effect) between the alignment layerand the liquid crystal material tends to orient the liquid crystalmolecule 50 so that the spontaneous polarization 56 of the liquidcrystal molecules 50 is directed to the inside. Therefore, when thesmectic layer 52 is formed, if voltages of +20 V, +7 V and 0 V areapplied to the gate electrodes 12, the pixel electrodes and the counterelectrode 34, respectively, as this preferred embodiment, thespontaneous polarization 56 of the liquid crystal molecules 50 isoriented so as to be parallel to the direction of the electric field asshown in FIG. 6(a), so that the quantity of light leaking out from theperipheral portion of the pixels can be decreased as small as possible.When the smectic layer is formed, even if voltages of +20 V, 0 V and −10V are applied to the gate electrodes, the pixel electrodes and thecounter electrode, respectively, as shown in FIG. 6(b), the spontaneouspolarization 56 of the liquid crystal molecules 50 is oriented in thedirection of the electric field, so that the same advantages can beobtained.

[0092] Furthermore, when the smectic layer is formed, if the voltagesare applied so that the direction of the spontaneous polarization 56 ofthe liquid crystal molecules 50 is different from the direction of theelectric field as shown in FIGS. 6(c) and 6(d), the quantity of lightleaking out from the peripheral portion of the pixels is larger thanthose shown in FIGS. 6(a) and 6′(b).

[0093] As described above, in this preferred embodiment, it is possibleto prevent light from leaking out from the peripheral portion of thepixels, so that it is possible to obtain a liquid crystal display havinga high display performance.

[0094] Furthermore, in this preferred embodiment, if the TFTs 18 arenegative TFTS, the direction of the spontaneous polarization 56 of theliquid crystal molecules 50 is different from the direction of theelectric field, so that the quantity of light leaking out from theperipheral portion of the pixels is larger than that in this preferredembodiment.

[0095] (Fifth Preferred Embodiment)

[0096] The fifth preferred embodiment of a liquid crystal displayaccording to the present invention will be described below. The liquidcrystal display in this fifth preferred embodiment has the sameconstruction as that of the liquid crystal display in the fourthpreferred embodiment, except that the liquid crystal material isdifferent.

[0097] In the same manner as that in the fourth preferred embodiment, anempty cell was formed. After this cell was put in a vacuum chamber to bein vacuum, A 10:1 mixture of a ferroelectric liquid crystal composition40 (phase series: solid phase→>30° C.→chiral smectic C phase→75°C.>nematic phase →>80° C.→isotropic phase, tilt angle: 22.5°,spontaneous planarization: 3 nC/cm²) and a UV-curable liquid crystal(UCL-001 produced by DAINIPPON INK & CHEMICALS, INC.) was injected intothe cell via a filling inlet. At this time, the cell and the liquidcrystal 40 were heated to 85° C. Thereafter, the filing inlet was sealedwith an epoxy adhesive.

[0098] Then, the extracted portions of the signal lines 16, gate lines12, auxiliary capacitive lines and counter electrode 34 of the cellfilled with the liquid crystal 40 were connected to terminals, to whichvoltages are to be applied, via an anisotropic conductive film. Then,the cell was heated to 77° C. in an oven. A voltage of +25 V was appliedto the gate lines 12 to cause the TFT elements 18 to be always in ONstate, and a voltage of +7 V was applied to the signal lines 16 to holdthe pixel electrode 15 at +7 V. In addition, a voltage of +7 V wasapplied to the capacitor lines, and a voltage of 0 V was applied to thecounter electrode 34. While these voltages are applied, the cell wascooled from 77° C. to 73° C. at a rate of 1° C./min to form a smecticlayer 52. In this state, the cell was irradiated with UV light (365 nm,10 mJ/cm²) to cure the UV-curable liquid crystal. Thereafter, no voltagewas applied, and the cell was cooled at a rate of 10° C./min.

[0099] After this cell was observed by a polarizing microscope, thesmectic layer 52 was the same as that shown in FIG. 10.

[0100] After the gap of this cell was measured, it was 2.0 μm. Thechiral pitch of the liquid crystal used in this preferred embodiment was4.0 μm which was longer than the cell gap. Therefore, the liquid crystaldid not have a twisted alignment.

[0101] Then, a set of polarizers 28 and 38 were applied on the outsideof the cell. Furthermore, the transmitting axis 38 a of one polarizer 38was parallel to the optical axis of the liquid crystal molecules 50 whenno voltage was applied, and the transmitting axis 28 a of the otherpolarizer 28 was perpendicular to the transmitting axis 38 a. On thecell on which the polarizers were applied, a driving circuit, such as adriver IC, was mounted, and a back light and so forth were mounted tocomplete a liquid crystal display in this preferred embodiment.

[0102] This liquid crystal display did not leak light out from thevicinity of the gate lines 12, so that it was possible to obtain acontrast of 200:1. In addition, the viewing angle(a region having acontrast of 10:1 or more and no reversal of gray scales) was 70° or morein vertical and horizontal directions, and the liquid crystal alignmentand the contrast were not deteriorated after driving tests at 0° C., 25°C. and 50° C. for 3000 hours.

[0103] In the fifth preferred embodiment, it is possible to obtain thesame advantages as those in the fourth preferred embodiment.

COMPARATIVE EXAMPLE

[0104] Referring to FIG. 7, a comparative example with the liquidcrystal display in the fourth preferred embodiment will be describedbelow.

[0105] First, a cell was formed in the same manner as that in the fourthpreferred embodiment.

[0106] Then, the extracted portions of the signal lines 16, gate lines12, capacitor lines and counter electrode 34 of the cell filled with theliquid crystal 40 were connected to terminals, to which voltages are tobe applied, via an anisotropic conductive film. This cell was heated to90° C. in an oven. A voltage of +20 V was applied to the gate lines 12to cause the TFT elements 18 to be always in ON state, and a voltage of0 V was applied to the signal lines 16 to hold the pixel electrode 15 at0 V. In addition, a voltage of +0 V was applied to the capacitor lines,and a voltage of +7 V was applied to the counter electrode 34. Whilethese voltages are applied, the cell was cooled from 90° C. to 25° C. ata rate of 1° C./min to form a smectic layer 52.

[0107] After this cell was observed by a polarizing microscope, thesmectic layer 52 was the same as that shown in FIG. 7. That is, thedirection of the smectic layer on the gate lines is greatly differentfrom that on the pixel electrodes, so that the smectic layer is bent.

[0108] After the gap of this cell was measured, it was 2.0 μm. Thechiral pitch of the liquid crystal used in this comparative example was4.0 μm which was longer than the cell gap. Therefore, the liquid crystaldid not have a twisted alignment.

[0109] Then, a set of polarizers 28 and 38 were applied on the outsideof the cell. Furthermore, the transmitting axis 38 a of one polarizer 38was parallel to the optical axis of the liquid crystal molecules 50 whenno voltage was applied, and the transmitting axis 28 a of the otherpolarizer 28 was perpendicular to the transmitting axis 38 a. On thecell on which the polarizers were applied, a driving circuit, such as adriver IC, was mounted, and a back light and so forth were mounted tocomplete a liquid crystal display in this comparative example.

[0110] This liquid crystal display leaked light out from the vicinity ofthe gate lines (between the gate lines and the pixel electrodes), sothat the contrast was 50:1. In a driving test at 0° C., the alignmentwas disturbed after 50 hours, and the contrast was lowered to 25:1.

[0111] (Sixth Preferred Embodiment)

[0112] Referring to FIGS. 8 and 9, the sixth preferred embodiment of aliquid crystal display according to the present invention will bedescribed below. The liquid crystal display in this sixth preferredembodiment has the same construction as that of the liquid crystaldisplay in the fourth preferred embodiment, except that switchingelements 18 have a structure wherein pixels are arranged upwards, that acolor filter is formed on an array substrate, and that no black matrixand no color filter are formed on a counter substrate.

[0113] The structure wherein pixels are arranged upwards is shown inFIG. 9. Gate electrodes 61 and capacitor lines (not shown) are formed ona glass substrate constituting an array substrate. The gate electrodes12 a and the capacitor lines are covered with a gate insulating film 62(see FIG. 9(b)). A semiconductor film 64 of an amorphous silicon servingas a channel is formed on the gate insulating film 62 so as to cover thegate electrodes 61 (see FIG. 9(b)). On the semiconductor film 64, achannel protective film 65 is formed. On the semiconductor film 64 onboth sides of the channel protective film 65, sources 66 a and drains 66b of an n⁺-type amorphous silicon are formed (see FIG. 9(d)). Thesources 66 a and the drains 66 b are connected to source electrodes 68 aand drain electrodes 68 b of a metal, respectively. The sourceelectrodes 68 a are connected to signal lines 16. On the sourceelectrodes 68 a and drain electrodes 68 b, a color filter 69 is formed.On the color filter 69, pixel electrodes 15 of ITO are formed. The pixelelectrodes 15 are electrically connected to the drain electrodes 68 bvia contacts 70 provided in the color filter 69.

[0114] In this preferred embodiment, no black matrix is arranged on thecounter substrate, and the switching elements have the structure whereinpixels are arranged upwards. In this structure, the pixel electrodesoverlap with each other on the gate lines and signal lines, so that itis possible to obtain a high aperture ratio. Similar to the fourthpreferred embodiment, a smectic layer was formed by the relationshipbetween voltages shown in FIG. 6(a). As a result, no alignment defectoccurred over the whole screen, and a front contrast of 300:1 wasobtained. In addition, the viewing angle (a region having a contrast of10:1 or more and no reversal of gray scales) was 70° or more in verticaland horizontal directions, and the liquid crystal alignment and contrastwere not deteriorated after driving tests at 0° C., 25° C. and 50° C.for 3000 hours.

[0115] While the TFTs 18 have been the positive TFTs in the sixthpreferred embodiment, the same advantages can be obtained if negativeTFTs are used.

[0116] As described above, according to the present invention, it ispossible to decrease the quantity of light leaking out from theperipheral portion of pixels as small as possible, and it is possible toobtain a liquid crystal display having a high display performance.

[0117] While the present invention has been disclosed in terms of thepreferred embodiment in order to facilitate better understandingthereof, it should be appreciated that the invention can be embodied invarious ways without departing from the principle of the invention.Therefore, the invention should be understood to include all possibleembodiments and modification to the shown embodiments which can beembodied without departing from the principle of the invention as setforth in the appended claims.

What is claimed is:
 1. A liquid crystal display comprising: an arraysubstrate including a plurality of scanning lines and a plurality ofsignal lines, said scanning lines and said signal lines being formed ona first substrate in the form of a matrix, a plurality of switchingelements which are formed at points of intersection between saidscanning lines and said signal lines, one end of each of said switchingelements being connected to a corresponding one of said signal lines,and each of said switching elements carrying out a switching action inresponse to a signal of a corresponding one of said scanning lines, aplurality of pixel electrodes, each of which is connected to the otherend of a corresponding one of said switching elements, and a firstalignment layer which is formed on said first substrate so as to coversaid pixel electrodes; a counter substrate including a counter electrodewhich is formed on a second substrate, and a second alignment layerwhich is formed on said second substrate so as to cover said counterelectrode; and a light control layer sandwiched between said arraysubstrate and said counter substrate, and including a liquid crystalmaterial having a spontaneous polarization and having a nematic phase oran isotropic phase on a high-temperature side of a chiral smectic Cphase, an optical axis of liquid crystal molecules in said light controllayer substantially staying when no electric field or a first electricfield of a first polarity are applied to said liquid crystal material,and said optical axis of said liquid crystal molecules responding inaccordance with a magnitude of a second electric field of a secondpolarity different from said first polarity when said second electricfield is applied to said liquid crystal material, wherein an electricfield between said scanning lines and said counter electrode has saidfirst polarity when said switching elements turn on.
 2. A liquid crystaldisplay as set forth in claim 1 , wherein said switching elements aredisposed under said pixels.
 3. A liquid crystal display as set forth inclaim 1 , wherein a direction of a smectic layer in said light controllayer has a distribution of 10° or less.
 4. A liquid crystal display asset forth in claim 1 , wherein each of said switching elements has anegative TFT, and said first alignment layer has an alignmentcharacteristic that the spontaneous polarization of liquid crystalmolecules is directed to said first substrate when no voltage is appliedto said liquid crystal material.
 5. A liquid crystal display as setforth in claim 4 , wherein said first alignment layer is made of apolyacrylonitrile or a polyimide.
 6. A liquid crystal display as setforth in claim 1 , wherein each of said switching elements has apositive TFT, and said first alignment layer has an alignmentcharacteristic that the spontaneous polarization of liquid crystalmolecules is directed to said second substrate when no voltage isapplied to said liquid crystal material.
 7. A liquid crystal display asset forth in claim 6 , wherein said first alignment layer is made of anylon or a benzocyclobutene polymer.
 8. A liquid crystal display as setforth in claim 1 , wherein the liquid crystal in said light controllayer has a tilted bookshelf structure.
 9. A liquid crystal display asset forth in claim 1 , wherein said liquid crystal molecules in saidlight control layer have an apparent tilt angle of substantially 22.5°or more.
 10. A liquid crystal display as set forth in claim 1 , whereina rubbing direction on said first alignment layer is anti-parallel to arubbing direction on said second alignment layer.
 11. A method forproducing a liquid crystal display comprising an array substrateincluding a plurality of scanning lines and a plurality of signal lines,said scanning lines and said signal lines being formed on a firstsubstrate in the form of a matrix, a plurality of switching elementswhich are formed at points of intersection between said scanning linesand said signal lines, one end of each of said switching elements beingconnected to a corresponding one of said signal lines, and each of saidswitching elements carrying out a switching action in response to asignal of a corresponding one of said scanning lines, a plurality ofpixel electrodes, each of which is connected to the other end of acorresponding one of said switching elements, and a first alignmentlayer which is formed on said first substrate so as to cover said pixelelectrodes; a counter substrate including a counter electrode which isformed on a second substrate, and a second alignment layer which isformed on said second substrate so as to cover said counter electrode;and a light control layer which is sandwiched between said arraysubstrate and said counter substrate and which is made of a liquidcrystal material having a spontaneous polarization and having a nematicphase or an isotropic phase on a high-temperature side of a chiralsmectic C phase, said method comprising: forming a chiral smectic Cphase with applying an electric field of a polarity between said pixelelectrodes and said counter electrode when a phase transition of saidliquid crystal material from a nematic phase or an isotropic phase tothe chiral smectic C phase occurs, wherein said polarity of saidelectric field equals to a polarity of an electric field between saidcounter electrode and said scanning lines when said switching elementsturn on.